After using StreetComplete for a while I noticed that the localization was incomplete and filed a ticket here. I was then informed that many of the preset strings come from iD editor which means we need to also translate all the iD editor presets into Thai. Please come and help out at https://www.transifex.com/openstreetmap/id-editor/translate/#th/presets/ if you can.
Also we’re coordinating volunteers through the OpenTech Thailand Telegram group so if you’re keen to join the community, please come along. https://t.me/+TE_R9c3j_8MT_1zW